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Thank you Mike I'm not sure how to install an adjustable needle in the piston, the guide rod is 3mm diameter so I would be working with M2 or M1.6 threads and I can't see how to lock that against vibration
Mike, The Stromberg CD carb had an interesting mixture enrichment for starting. There was a rod with a flat on it as the manifold side of the bridge which was rotated to lift the piston and increase the fuel orifice as well as restricting the air flow.
I think that if I can adjust the jet height with a fine thread (0.5mm pitch or less) it will be easier that putting a hex key or watch makers screwdriver down the piston guide.